Anthropic launches Claude Fable 5.1 on the Claude API
Claude Fable 5.1 is the successor to Claude Fable 5 for long-running agentic coding, knowledge work and research. Claude Mythos 5.1 launched alongside it for Project Glasswing participants.
Why it matters
Code that forces tool use breaks on Claude Fable 5.1, because the tool_choice types any and tool return an error.